When a display apparatus works in a condition of high light intensity, e.g., under bright illumination or in the open air, outside light is reflected by the surface to cause reflection of ambient objects in the display screen. This tends to make the image hard to watch, in particular a dark portion of low light intensity. Various attempts have been made to solve these problems, e.g., coating the display screen with an anti-reflecting membrane to reduce light reflection by interference, or roughening the screen's outermost surface to reduce regular reflection intensity by scattering.
However, roughened surface causes light scattering, which makes the image slightly vague. A display screen not surface-roughened tends to show a clearer image for a personal computer which frequently displays letters and TV set which frequently shows pictures. Plates on monitor surfaces are generally made of transparent materials, e.g., glass (refractive index: 1.5 to 1.54), acrylic resin (refractive index: 1.49) and polyethylene terephthalate resin (refractive index: 1.56).
Reflectance R of these members is given by the following Equation (1):R={(n1−n0)/(n1+n0)}2  (1)wherein,n0: refractive index of airn1: refractive index of the memberEquation (1) can be reduced to Equation (2), knowing that refractive index of air n0 is generally 1.0.R={(n1−1)/(n1+1)}2  (2)
The transparent member of glass, acrylic resin or PET resin has a reflectance of 3.9 to 4.0, 3.9 or 4.8%, found by substituting n1 in Equation (2) by a refractive index of each material. The member coated with a single-layer anti-reflecting membrane of adequate thickness to reduce the reflectance has a reflectance R′ given by Equation (3):R′={(n22−n0×n1)/(n22+n0×n1)}2  (3)wherein,n0: refractive index of airn1: refractive index of an outermost surface platen2: refractive index of a membraneEquation (3) can be reduced to Equation (4), knowing that refractive index of air n0 is generally 1.0.R′={(n22−n1)/(n22+n1)}2  (4)Reflectance R′ should be theoretically zero, when n22=n1 (or n2=√n1).
This means that a suitable refractive index of an anti-reflecting membrane on glass is around 1.22. It is however very difficult to secure a sufficiently low refractive index by a single-layer membrane, because even a fluorinated resin and magnesium fluoride have a respective refractive index of around 1.34 and 1.38, the former being known as a material of relatively low refractive index and the latter as an inorganic material of particularly low refractive index.
Recently, methods for decreasing refractive index of a single-layer membrane have been proposed. For example, Patent Document 1 proposes a thin membrane of aerogel. It is composed of fine particles having pores inside (fine hollow particles) and a binder for supporting the fine hollow particles. The inside pore has a refractive index substantially the same as that of air (refractive index: 1.0), with the result that the membrane as a whole has a refractive index close to that of air even when the fine hollow particles or binder has a high refractive index. In other words, the member can have a reduced refractive index by forming this membrane on a plate.
Patent Document 2 proposes another method for decreasing refractive index of a single-layer membrane without using aerogel. It is composed of superfine organic particles having surfaces exposed in the area near air and roughened, to decrease surface density and thereby to decrease membrane refractive index.
Patent Document 3 proposes still another method for providing a low-refractive-index membrane having pores of honeycomb structure. These pores are designed to pass through the fine silica particles and run in parallel to each other, in order to maximize void fraction without damaging strength of the fine silica particles. It claims that a low-refractive-index membrane of high strength can be produced.    Patent Document 1: JP-A 2003-201443    Patent Document 2: JP-A 7-92305    Patent Document 1: JP-A 2004-83307.